He’d set reminders for himself on his calendar, but found this mechanism wasn’t ideal, since it was tedious to modify every calendar entry to reflect communications with the contacts he had already heard back from.

“I wished there was a way my calendar knew that he or she responded,” he says.

So in July 2007 Keller built himself a Web-based software tool that bypassed his calendar entirely, instead sending e-mail reminders about people he needed to communicate with, and automatically deleting those reminders once they were knocked off the to-do list. Last spring, he started mentioning the tool to a few friends who have similar “e-mail woes,” and what started out as a personal project has since evolved into a part-time business endeavor with more than 1,000 users.

Keller’s creation, FollowUp.cc, is built around a set of e-mail addresses that function as reminders. Users can add a FollowUp e-mail address in the CC or BCC line of an e-mail when they want to remind themselves to reconnect with the recipient at a later date.

So far, the development of the tool has “all been organic,” says Keller, who also works full-time at the Cambridge, MA-based Web marketing startup, HubSpot, as a “labs product owner,” working on experimental projects to help launch client companies’ websites. “I’m never thinking it’s done or polished enough. As we’ve learned in startups, you just need to get it out there and deal with it.”

Keller had a few brief months to focus on FollowUp between the time he left Fafarazzi in July 2009 and his hiring at HubSpot in late September 2009, but the system is now back to side-project status. He investigated potential business models (more on that in a minute) for FollowUp, but intends to remain full-time at his HubSpot job.

FollowUp frames reminders using four different time references. Users can remind themselves to follow up at a specific length of time into the future, such as three hours from now. In other words, to send a note that will automatically show up in your e-mail inbox two days from now, you’d send a copy to 2d@followup.cc. Users can also select an upcoming day of the week and a time on that day—same thing with a date in the future (Mar30@followup.cc, for example). Lastly, for the more immediate tasks, users can set a reminder for a certain time in the next 24 hours (i.e. 5pm@followup.cc).